Effects of Vacancies on the Physical Properties of Platinum

Most of those who work with metals at high temperature think of vacancies (vacant lattice sites) as important only in the context of diffusion. It is usually left to the solid-state physicist to study the effects of vacancies on such physical properties as thermal ex-pansion, enthalpy and electrical resistivity, and even he may be interested largely in what he can learn about the vacancies them-selves. Of course, whereas diffusion is a process controlled by vacancies, vacancies merely influence these other properties to a limited extent. Nevertheless, the influence can be sufficient to be of direct concern to those who use platinum as a high-temperature reference material. The Size of Vacancy Effects As is well known, the equilibrium mono-vacancy concentration C(T) in a metal is an exponential function of the absolute tempera-ture T: C(Tj =exp(S’/k)exp (-EF/kT) where C is the atomic fraction of vacancies, SF is the monovacancy formation entropy, E ” is the monovacancy formation “energy” (enthalpy), and k is Boltzmann’s constant. Because of this, at temperatures approach-ing the melting point T, the variation of C with temperature is so rapid that, although the effect of vacancies on, say, the electrical resistivity may be quite small (c I per cent), the effect on the corresponding dzfferential property-the temperature coefficient of resistivity-is an order of magnitude larger at least. To make precise estimates of vacancy effects in platinum is not altogether easy. The “traditional ” experiment (I) of accurately I